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This invention relates to electrical components and, more particularly, to semiconductor devices, such as Schottky diodes, which include surge guard protection material to inhibit damage from electrostatic discharge.
The use of hand held and desktop electronic devices, such as computers and mobile telephones, has increased tremendously in recent years, and the demand for these devices continues to increase at an overwhelming rate. These devices by nature are generally portable or easily accessible and are frequently in direct contact with users. Further, they are exposed to dust and may be carried outside the office increasing their exposure to the elements. This operational environment presents a risk of damage to the electrical components of these devices from transient threats such as electrostatic discharge (ESD) and lightning.
In Schottky diodes, two layers of preferably N-type conductive material are provided with a rectifying barrier metal. The two layers include an epitaxial layer grown on a highly doped, single crystal semiconductor substrate layer. The rectifying metal is deposited on the epitaxial layer to form a semiconductor junction which provides the rectifying property of conducting current in one direction when the barrier metal is under positive bias and blocking current flow when the barrier metal is under the opposite bias. In the case of Schottky power rectifiers, the dominant type of current across the barrier is the thermionic emission current.
In a subsequent processing step, a back, metal contact is formed on the N+ substrate layer side of the diode. The function of the metal contact is to conduct current with minimal resistance in either direction, that is, under any bias applied to the diode. This is primarily achieved by the high doping level of the N+-type substrate, which makes field emission or tunneling a prevalent mechanism of the current flow through the metal contact.
To achieve acceptable reverse bias blocking characteristics and protect the diodes from transient threats, various methods have been developed for termination of the periphery of the barrier metal. In one common method of inhibiting damage from ESD, lightening, load switching and other transient threats, a perimeter ring of P+-type material is diffused into the xe2x80x9ctopxe2x80x9d Nxe2x88x92-type conductive layer around the perimeter termination of the barrier metal. The P+-type ring minimizes reverse current leakage and provides a reverse current path for reverse current surge to protect the barrier metal and N-type conductive layers from exposure to critical electrical fields generated by transient threats and during electrical testing. That is, the avalanche breakdown voltage is lowest at the P/N junction formed by the P-type ring, so that when ESD occurs sufficient to exceed the avalanche breakdown voltage at the P/N junction, the P-type ring directs the reverse flow of current through the P/N junction and away from the central area of the barrier metal.
The protection provided by the P-type ring is effective until the reverse voltage drop across the P/N junction rises to the breakdown voltage of the Schottky diode or the reverse current causes localized heating of the barrier metal adjacent the P-type ring; it is not definitively known which affect is controlling. When the reverse current reaches a sufficiently high level, the barrier metal and xe2x80x9ctopxe2x80x9d N-type layer are melted rendering the diode inoperable for its intended function. To increase the effectiveness of the P-type guard ring, a variety of guard ring features are optimized to find the best combination of features and enhance reverse surge capability. For example, the width and/or the depth of the ring can be increased, the concentration of dopant in the ring can be increased, and/or the thickness and resistivity of the xe2x80x9ctopxe2x80x9d N-type layer can be reduced. While optimization of these features achieves incremental improvements in reverse surge capability, continuous progress in the electronics area and higher consumer expectations of product reliability and performance have prompted higher reverse surge performance standards, which are nearly beyond the incremental improvements previously achieved. Further, modifying the diode in these ways can undesirably degrade the operational parameters of the diode.
There is, therefore, provided in the practice of the invention a novel electrical component with a distributed reverse surge guard, which increases protection from reverse current surges without degrading operational parameters of the electrical component. The electrical component broadly includes a conductive pad with a primary conductive area edge. A first region of surge guard material is positioned adjacent the primary conductive area perimeter edge, and a second region of surge guard material is positioned adjacent the conductive pad and spaced from the primary conductive area edge.
In a preferred embodiment, the second region has first and second sides, which are positioned within the primary conductive area edge. Preferably, the first region is a substantially continuous perimeter loop of P+-type material, and the second region is a grid of perpendicular P+-type material lines extending between opposite sides of the perimeter loop. The grid forms a plurality of substantially rectangular, preferably square, inner loops of substantially the same size extending within the perimeter loop, and some of the inner loops overlap in part with the perimeter loop.
It is further contemplated in the practice of the invention, that the distributed reverse surge guard is used in a diode having first and second conductive layers with conductive types and with the conductive pad in electrical communication with one of the conductive layers. At least a single region of surge guard material including a first side and a second side is positioned adjacent the conductive pad with each of the first and second sides within the perimeter of the primary conductive area of the conductive pad.
It is still further contemplated in the practice of the invention that the diode is constructed by forming a monocrystalline substrate layer of semiconductor material having a first conductive type and depositing an epitaxial layer on the substrate. The epitaxial layer having a second conductive type. A first region of surge guard material is formed on an outer surface of a selected one of the substrate layer and the epitaxial layer, and the first region has extremities and an opposite conductive type. A second region of surge guard material is also formed on the outer surface of the selected layer. The second region is also of the opposite conductive type and is positioned within the extremities of the first region. A conductive pad is also placed over the outer surface of the selected layer In a preferred embodiment, the conductive pad on the outer surface of the selected layer, covers at least a portion of the second region and is deposited on the outer surface after the first and second regions are formed by diffusing a P-type dopant into the epitaxial layer. Preferably, the conductive pad covers all of the second region and at least part of the first region. In constructing the diode the first and second regions are formed substantially simultaneously, and the conductive pad is deposited after the first and second regions are formed.
Accordingly, it is an object of the present invention to provide an improved electrical component with a distributed reverse surge guard providing increased protection from ESD and other transient threats.
It is a further object of the present invention to provide an improved method of constructing an electrical component with a distributed reverse surge guard providing increased protection from ESD and other transient threats.